The invention relates to a force meter for measuring forces or stresses in general, and relates more particularly to a force meter comprising an optical waveguide integrated in a substrate propagating two perpendicular polarization modes and whose refractive indexes vary in response to the application of a force, thus introducing a phase difference between the modes propagated which is proportional to the applied force.
It has already been proposed in Dandliker et al. Swiss Patent No. 661985 to use a birefringent single-mode optical fiber to measure a force applied thereto. The fiber is wound into the form of a helix, the turns of which are arranged between two plates. The application of a force to one of the plates with respect to the other induces a variation in the effective indexes of the polarization modes within the fiber. This photoelastic effect has, moreover, already been described in an article by Scott C. Rashleigh "Origins and Control of Polarization Effects in Single-Mode Fibers" (Journal of Lightwave Technology, Vol. LT-1, No. 2, June 20, 1983, pp. 312-331). In the above-mentioned Patent, the detection by interferometric measuring between polarization modes is effected with the aid of discrete optical elements which introduce a phase shift between the waves propagated by the fiber.
The idea of using an optical fiber as a transducer creates a certain number of disadvantages, of which the following should be mentioned.
The load transmitted through the intermediary of the plates is not directly and wholly applied to the core of the fiber. Between the core, which constitutes the active medium, and the plates there exists an intermediate medium consisting of the cladding of the fiber, the presence of which diminishes the action of the force on the core.
Further, the use of an optical fiber restricts the choice of fibers to only those comprising a metal or ceramic jacket, because the use of fibers with plastic jackets induces hysteresis, a creep effect and a variation in the force bearing surface. Although ideal, the use of a jacket-free fiber cannot be contemplated, due to its short life span.
Still further, the obtainment of perfect contact between the fiber and the plates requires both a high level of uniformity in the diameter of the fiber used and very flat plates, which thus result in high production costs.
Moreover, the conventional optical detection device described in the above-mentioned Swiss Patent comprises several discrete optical elements such as a quarter-wave plate and a half-wave plate, a polarizer and detectors. Such a device, having recourse to a configuration of discrete components, is difficult to miniaturize.
Finally, the problem arises of immobilization of the fiber between the plates. It is necessary that the fiber be protected, its fixation must not induce any parasitic birefringence, it must be in contact with the plates over all the interaction length, and the force must be transmitted thereto in a uniform manner. The Swiss Patent does not provide any element of a solution to this problem.